Systems and methods that supply power without electrical wiring are sometimes referred to as wireless energy transmission (WET). Wireless energy transmission greatly expands the types of applications for electrically powered devices. Implantable medical devices typically require an internal power source able to supply adequate power for the reasonable lifetime of the device or an electrical cable that traverses the skin.
More recently there has been an emphasis on systems that supply power to an implanted device without using transcutaneous wiring, sometimes referred to as a Transcutaneous Energy Transfer System (TETS). Frequently energy transfer is accomplished using two magnetically coupled coils set up like a transformer so power is transferred magnetically across the skin. Conventional systems are relatively sensitive to variations in position and alignment of the coils, typically requiring the coils to be physically close together and well aligned.
Existing systems that transmit power wirelessly based on magnetic fields typically operate in the near-field only, where the separation of the transmitter and receiver coils is less than or equal to the dimension of the coils.
Wireless powering has long been of interest for enhancing the function of implantable electronics, beginning in the early 1960's with experiments in transporting electromagnetic energy across the chest wall. Drawing conceptually on schemes for transferring power over air through objects coupled in the near-field, early manifestations involved bulky coils tether to vacuum tube power supplies or battery cells that posed severe challenges for long-term operation in the body. Advances in semiconductor technology have since enabled sophisticated devices that incorporate sensing and stimulation capabilities within cellular-scale dimensions. Nearly all existing systems, however, continue to require large structures for energy storage or harvesting, often several centimeters in the largest dimension with overall size, weight, and efficiency characteristics that constrain opportunities for integration into the body.
Near-field approaches rely on strong coupling occurring between objects with matched electrical characteristics, such as resonances and impedances. These near-field approaches do not generalize easily to geometries with extreme size asymmetry, while far-field transfer is limited by absorption over surfaces of the body.
When electromagnetic radiation is focused from air into a material such as tissue, refraction at the air-material interface determines the diffraction limit. Conventional lenses, placed in the far field of the interface, control only propagating wave components in air. As a result, their focusing resolution in material is diffraction-limited at the free-space wavelength λ because higher-wave-vector components in material cannot be accessed by far-field light. These high-wave-vector components correspond to plane waves propagating at angles greater than the critical angle, which are trapped in the material by total internal reflection.
A critical or “forbidden” angle is it is the largest angle of incidence for which refraction can still occur. Prior optical methods for accessing these “forbidden” angles of refraction rely on solid immersion lenses: semispherical domes of high-index material placed at or near the air-material interface. This capability enables radiation to be focused to a spot much smaller than the free-space wavelength, with a diffraction-limited resolution set by the material wavelength ˜λ/n.
In the radio-frequency regime, active devices have also been used to focus radiation from air into material. These systems generally involve phased arrays, in which many active radio-frequency components are used to generate the prescribed spatial phase profiles. These components can be bulky and require additional power supplies.